Heat sinks for dissipating heat generated by electronic device such as semiconductor chips or semiconductor chip packaging substrates such as used in electronic computers have a substantial history. A driving force behind continued investigation of improved heat sinks is to dissipate more heat per unit area generated by advanced very large scale integrated semiconductor chips which contain increasingly more heat generating circuits on a given area of the chip. Concomitantly, of equal importance, a driving force for continued investigation of heat sinks for semiconductor chips is reduction in the cost of fabrication of the heat sink to reduce the cost of the packaged semiconductor chip and correspondingly the cost of the electronic computer. Traditional heat sinks are commonly made from extrusion, casting and machining of materials most typically metals such as aluminum. Aluminum is a relatively inexpensive metal having a relatively high thermal conductivity. A machine aluminum heat sink is made from a block of aluminum from which portions are machined away. Machining is a time consuming and a labor intensive process which also does not maximally utilize the material. In extrusion, material is pushed through a form to make the shape of the desired heat sink. Extrusion requires costly machinery, and requires relatively large amounts of material to form the extruded part. In casting, molten material is poured into a cast having the shape of the desired part. Casting is an extensive labor intensive process which also utilizes more material than necessary to form the part.
Heat sinks generally have fins which are typically substantially planar members projecting away from a base. The fins do not have to be limited to this shape. Fins resulting from machining, extrusion and casting are not optimized thermally. Due to the nature of and natural limitations of machining, extrusion and casting the fins are typically thicker than a minimal thickness needed to be an effective heat dissipation means. The fins made by these methods generally have a thickness greater than about 1 mm. Moreover, the aspect ratio of the fins that is the ratio of the fin height to the thickness, is generally limited in these fabrication methods to less than about 20.
A fluid which can be a gas or a liquid, but typically air, is generally directed across the fins or between the fins to extract heat therefrom. When a gas is used there is generally a pressure drop at and between the heat sink fins as compared to the pressure at the output of the gas stream. This pressure drop is caused by the fins and the rest of the heat dissipation assembly providing resistive barriers to the fluid flow. This is generally caused by the following structure dimensions: the fin thickness, the fin height and the spacing between fins. The fin thickness provides a hard obstruction to the fluid flow; so does the fin height. As the spacing between the fins becomes smaller, there is less room for the fluid to flow between the fins; therefore, increasing the fluid flow resistance resulting in a decrease in the fluid pressure between the fins. In order to boost the cooling power of the heat sink while keeping the air drag low, thinner and higher aspect ratio fins need to be used.
By the traditional methods of machining, extrusion and casting it is difficult and expensive to make fins which are thin and have a high aspect ratio. Alternative methods to achieving high aspect ratio fin assemblies is to glue, to braze or to solder high aspect ratio sheets (fins) of thermally conducting material to a base. These methods, however, are costly since they require a relatively thick base material plate to hold the fins, relatively costly adhesive materials and relatively high assembly cost. In such a technique grooves are typically formed in the relatively thick base material into which the fins are inserted and flued, brazed or soldered thereto. Furthermore, the fin aspect ratio is also limited in these heat sinks by the dimensional instability of the thin fins. High aspect ratio fins having a minimum thickness are difficult to handle since they lack rigidity.
For any one of the above traditional types of heat sinks, the planarity of the heat sink surface which comes into contact with the heat generating surface will affect the thermal resistance therebetween. If the heat sink has a relatively large base which is not machined to a high degree of planar flatness and if the surface to which it is to be thermally connected to is not machined to a high degree of planar flatness, there will be substantial regions wherein there will be spaced between the heat sink base and the heat generating surface because of the lack of perfect flatness of each side. These spaces provide regions of high thermal resistance.
Applicants have discovered a heat dissipating structure which avoids the substantial costs of machining, extrusion, casting and adhesively attaching fins to a thick base plate by using folded sheets of thin sheet material. The heat dissipating structures of the present invention also avoid the problem of the lack of perfect planarity between the heat sink base and the heat generating surface. The present invention uses a design wherein fins or a fin assembly (group of fins) are slidably engaged with respect to an adjacent fin or fin assembly to permit the fin or fin assembly to be pressed into intimate thermal contact with the heat generating surface and to thereby have a substantially improved conformal and thermal conduction thereto.
The structures of the present invention have fins for dissipating heat which have a minimum thickness and a high aspect ratio and a minimum spacing therebetween.
It is an object of the present invention to provide a heat dissipation structure which can dissipate heat in excess of 4.0 watts per square centimeter by blowing air over the fins at a pressure of 18.0 N/m.sup.2.
It is another object of the present invention to provide a heat dissipation structure which is substantially inexpensive to fabricate.
It is another object of the present invention to provide heat dissipation structures having fin assemblies wherein adjacent fin assemblies have parts thereof which are slidably engaged to each other to permit each assembly to be urged in substantially conformal thermal connection to a heat generating surface.
It is another object of the present invention to enclose the assembly of fins with a housing providing a duct-like structure to direct a fluid to pass over and between the fins by directing a fluid shown along a direction parallel to said fins.
It is another object of the present invention to provide a bias means between the housing the fins to urge the fins in conformal contact with the heat generating substrate.
The prior art structures described below show resilient sheet material folded into a serpentine or corrugated shape which is held between a duct-like-cap and a heat generating surface. The prior art structures however, do not have an optional thermal contact between the serpentine structure the heat generating surface. Some of these prior art serpentine structures are typically curved at the region of contact with the heat generating surface thereby having a relatively small region over which heat can transfer from the heat generating surface into the serpentine fin structure having flat regions which only cover approximately half of the heat generating surface. In contradistinction, the structures of the present invention have fin assemblies with bases which substantially completely cover the heat generating surface providing an optimal thermal connection between the heat generating surface and the fin assemblies. Moreover, since the fin assemblies of the present invention are permitted to move independently of each other there is achieved intimate thermal contact between the fin assemblies and the heat generating surface to substantially enhance the heat dissipation power of the structures of the present invention.